U.S. Geological Survey - Hawaiian Volcano Observatory
Volcanic Hazards - Types - Lava Zones
When lava pours into the ocean at high rates from a lava-tube entry, beautiful and spectacular explosions called tephra jets (above) commonly occur. With temperatures higher than 1,100 degrees Celsius, lava can instantly transform seawater to steam, causing explosions that blasts hot rocks, water, and molten lava fragments into the air. In general, the more intense the incoming waves, the more energetic the tephra jets. The incoming waves disrupts the lava exiting the tube and increases the surface area of the molten stream that is exposed to seawater by more than 10 to 20 times.
The most violent and dangerous steam-driven explosions, however, occur when the leading edge of a growing lava delta suddenly subsides or collapses into the sea. The resulting disruption of the lava tube system within the delta forces seawater to mix with lava and hot rock surrounding the tube system in a confined environment.
For people standing on the delta or it's leading edge, these sudden steam-driven explosions can be fatal.
A lava delta grows seaward as lava enters the sea and builds a foundation of loose lava fragments on the submarine slope. The platform of debris is subsequently capped by pahoehoe lava flows. In this sketch, the delta's leading edge has reached the steeper submarine slope and has started to subside, forming a lava bench. Lava tubes at the edge of the delta can reside below or at sea level, due to the continuous and sometimes abrupt subsidence of the delta and bench. As long as the tubes remain intact and water does not gain ready access to the tube system, lava entering the sea will not generate significant explosions.
Bubble bursts and littoral lava fountains may occur when the leading edge of a delta subsides but doesn't slide away completely, which thereby allows seawater to infiltrate the lava tube system.
Since a growing delta may collapse or subside at any time and the intensity of any one type of explosion may change suddenly, a growing lava delta is hazardous and should only be viewed from behind the former sea cliffs.
A tephra jet is by far the most likely type of explosion a visitor is likely to witness after a bench collapse has severed an active lava tube. Waves rushing against the open stream of lava "explode" in a cloud of steam, hot water, molten spatter, and tephra fragments. Where the spatter and tephra fall onto the delta's leading edge, a semicircular littoral cone a few meters tall usually forms.
Visitors often walk on the sides of active littoral cones, unaware that the intensity of explosions can increase at any time or that the cone could collapse into the sea without warning.
Collapse of a growing lava delta exposes extremely hot newly solidified lava flows within the delta to seawater, triggering a type of steam explosion we call a blast. The explosive blast shatters the solidified lava flows of the delta into fragments and hurls them as far as 200 m inland! A collapse event on April 19, 1993, swept a man into the sea and generated a relatively large blast that showered lava rocks 25 to 110 cm (10 to 45 in) in diameter over an area equal to about 3 football fields.
Littoral Lava Fountains
Spectacular and rare, this type of lava-seawater explosion produces fountains of molten lava and steam that reach heights of more than 100 m. The explosions of molten spatter, bombs, and smaller tephra fragments quickly build a circular cone on the subsided lava delta, sometimes in a matter of minutes. Originating from deeper within the subsided delta and closer to the shoreline than bubble bursts, littoral lava fountains are much more energetic and dangerous.
Bubble bursts are characterized by sporadic bursts of molten, dome-shaped lava sheets emanating from a circular rupture in the roof of a tube a few meters inland from the shoreline. Individual bubbles can reach diameters of 10 m in less than 2 seconds before they burst. The bubble fragments continue on their radial trajectories for up to 10 m more before falling to the ground. At the end of a burst, a pool of lava that remains in the roof of the lava tube gradually drains away. These bursts are frequently accompanied by a loud boom that shakes the entire delta.
Mattox, T.N, 1993, Where lava meets the sea: Kilauea Volcano, Hawai`i: U.S. Geological Survey Earthquakes and Volcanoes, v. 24, n. 4 p. 160-177.
Mattox, T.N, and Mangan, M.T., 1997, Littoral hydrovolcanic explosions: a case study of lava-- seawater interaction at Kilauea Volcano: Journal of Volcanology and Geothermal Research, v. 75, p. 1-17.